RISM is an index and not a collection, so it provides you with listings of objects and not the objects themselves. In this case, the objects are primarily scores, though there are also some treatises and libretti included in the index.
There's a very good chance that you won't need to use RISM as often as other music research tools, like Grove and RILM. But if you need to locate music manuscripts, there is no other tool that will allow you to cross search the holdings of music libraries to find comprehensive results - RISM is it. (WorldCat does not contain the breadth of entries that RISM does when it comes to music manuscripts).
Oh, and one more thing: RISM is one of the few open sources for music research on the web. RILM, Music Index, IIMP, Grove, JSTOR, etc. are all behind a paywall, which means you have to subscribe to access the content.
RISM is free to everyone, all the time. Enjoy.
RISM records contain incipits, or short musical excerpts that are useful for identifying that a given record is a match without having to see the manuscript itself.
What's even more spectacular is that you can actually search the incipits using RISM's keyboard search in the advanced search screen.
SEARCH BY MELODY
RISM's now has a melody search tool that allows you to plunk out a tune on a digital keyboard in order to locate music.
ENGLISH AVAILABLEIn the upper right corner of the screen, you can toggle RISM's search between German and English. However, this just translates the search and the facets - not the results lists. Results will be written in the language in which the item was handwritten or published.